1. Field of Invention
The present invention relates to a projection display apparatus that separates light emitted from a light source into color beams of at least two colors, modulates these color beams via light valves formed of liquid crystal panels according to image information, synthesizes the modulated color beams again, and projects and displays the synthesized beams onto a projection plane via a projection optical system. More particularly, the present invention relates to a dichroic prism capable of correcting relative displacements of projection pixels of the colors caused by chromatic aberration of magnification.
2. Description of Related Art
A well-known type of projection display apparatus includes a light source, a color separation device that separates a light beam emitted from the light source into color beams of three colors, three light valves for modulating the separated color beams according to image information, a dichroic prism serving as a color synthesizing device that synthesizes the color beams modulated by the three light valves, and a projection optical system for enlarging and projecting the beams synthesized by the dichroic prism onto a projection plane.
The dichroic prism is disclosed in, for example, Japanese Unexamined Patent Publication No. 7-294845. As shown in FIG. 1, a dichroic prism 1000 disclosed in this publication is shaped like a quadrangular prism as a whole by bonding four rectangular prisms 11, 12, 13, and 14 having a cross-section shaped like a rectangular isosceles triangle. In this dichroic prism 1000, red reflecting dichroic planes 1500 and 1500 and blue reflecting dichroic planes 1600 and 1600 diagonally extend along the junction surfaces of the prisms 11-14 so that they form a flat red reflecting dichroic plane and a flat blue reflecting dichroic plane, respectively. A layer of an adhesive 1800, which is formed between the junction surfaces to join the rectangular prisms 11-14, is 10 .mu.m to 20 .mu.m thick.
Such a dichroic prism 1000 is used as a color synthesizing device in a projection display apparatus. In this case, color beams R, G, and B from liquid crystal light valves 925R, 925G, and 925B, which are placed to oppose three sides of the dichroic prism 1000, are synthesized by the pair of dichroic planes 1500 and 1600, and the synthesized beams are emitted from the other side of the dichroic prism 1000 toward a projection optical system.
Since the influence of chromatic aberration of magnification on images of the colors, which are projected and displayed via the projection optical system, increases with the distance from the center, displacements of the projection images of the colors become large. For example, as shown in FIG. 12, the areas of color projection images projected on a projection plane 100, a blue projection image B', a green projection image G', and a red projection image R', increase in this order. In terms of projection pixels, projection pixels b, g, and r of the colors are not in complete alignment, as shown in FIGS. 13(A)-13(C), and there is a pixel displacement d1. While the pixel displacement d1 hardly affects display quality when it is small (d1.ltoreq.d), it degrades the quality of a projection image when it exceeds the permitted limit (d.ltoreq.d1=d2). The sizes of the color projection images R', G', and B' vary with the material of the projection optical system. The red projection image R' is not always the largest, and the areas of the projection images of the colors relatively differ from one another.
In the industry associated with projection display apparatuses, efforts have been made to increase the definition of a light valve formed of a liquid crystal panel and the like by increasing the number of pixels of the light valve. There has been a tendency to reduce the area of each pixel with this increase of definition, which lowers the permitted limit of the above-described pixel displacement d1 and decreases the tolerance for chromatic aberration of magnification. Therefore, there is a need to reduce the influence of chromatic aberration of magnification on a projection image by correcting chromatic aberration of magnification, in order to obtain a high-resolution and high-quality projection image.